In today's business and scientific world, color has become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of digital color imaging systems are continuously looking for ways to improve the total image quality of their products. One of the elements that affects image quality is the ability to consistently produce the same quality image output on a printer from one day to another, from one week to the next, month after month.
Another element is the ability to accurately capture colors on an image acquisition device such as a scanner or digital camera. In particular, color scanners are an essential component in the digitization of color hardcopy documents. High-quality color scanning requires that the scanners are accurately characterized with respect to standard measurable descriptions of color. Furthermore, since color scanners are commonly available in many imaging environments, they can be used to measure color for printer calibration, characterization and/or diagnostics purposes. Again this requires accurate color characterization of the scanner.
Standard approaches for scanner color characterization are carried out as follows [REFERENCE: Raja Bala, “Device Characterization”, Chapter 5 of Digital Color Imaging Handbook, Gaurav Sharma Ed., CRC Press, 2003]. First a target comprising color patches is printed and scanned. The target is simultaneously measured with a color measurement device to obtain spectral reflectance or calorimetric measurements such as CIELAB for each of the printed patches. Scanner characterization is the process of relating the scanned device-dependent (usually RGB) signals to the spectral or calorimetric device-independent representation using the target patch data. The scanner characterization can be implemented either with a series of analytic functions (e.g. matrices, polynomial, etc) or with 1-dimensional and/or multidimensional lookup tables (LUTs). These functions or LUTs are stored in a scanner profile.
It is well known that the scanner characterization is very closely tied to the hardcopy medium and colorants being scanned. This means the scanner characterization process must generally be repeated for each input medium (i.e. combination of substrate, colorants, and image path elements). Thus, for example, different scanner color characterization profiles are required for use in scanning prints made with a photographic versus electrophotographic versus inkjet printing system. The primary reason for this is that color scanners are, in general, not calorimetric, so that the relationship between the response of the scanner and that of the human eye changes in a nonlinear fashion depending on the spectral properties of the medium being scanned. This property is referred to as scanner metamerism.
Many output devices render color via a halftoning process, which prints dot patterns on the medium. In addition to the aforementioned dependence of the scanner characterization on media and colorant properties, it turns out that the scanner color response can also be a strong function of the characteristics of the halftone used to generate the color prints. This means that if the halftoning method used to create the scanner characterization target is different from the halftoning method used to produce the hardcopy images that are ultimately scanned; this can produce undesirable errors in the scanner color correction process. Said differently, if the scanner profile is trained on one halftone, it may produce unacceptable errors when scanning prints made with a different halftone. The characteristics of the halftone that the scanner characterization is particularly sensitive to are screen frequency and dot growth and overlap characteristics (e.g. clustered vs. dispersed or stochastic). This phenomenon is referred to herein as halftone metamerism. As such, it limits the accuracy of the scanner to predict color from a hardcopy printed using a halftoning method different from the one that was used to derive the scanner profile.
Thus, there is a need for a scanner characterization technique that is “halftone-independent”, or equivalently robust across a wide variety of halftones, thus overcoming the halftone metamerism problem. Clearly, using a target that comprises some finite set of halftoning schemes is somewhat impractical due to the vast variety of halftoning methods that can be employed. The alternative strategy of supporting different scanner profiles for different halftones now places a burden on the user to correctly associate the correct halftone with the correct profiles. To mitigate these problems, the use of a single halftone-independent target for scanner characterization is proposed.